Your experience with speed boosters/focal reducers/accelerators

[Zang]

Hi all,

What is your experience with those devices I mentioned? I refer to the names as generic equipment class, not any specific makers. Optical design is not an easy task and I am a bit surprised by claims that the boosters have next to none sharpness impact, especially when they are designed to accommodate a vast number of lenses that they can be used with.

Cheers,
Zang 

[HaraldH]

I use a Viltrox 0.71x Nikon F to m43 adapter. I have not done any systematic investigation, but I have not noticed any adverse impact on resolution compared to a 'plain' K&F adapter. I have used a variety of Nikkors: 24 mm /2.8 'K', Nikkor-S 5 cm/2, 55 mm/3.5 micro, 105 mm/2.5 Sonnar type and Nikkor Q 200 mm/4.0.

I am not an optical engineer, but I suspect it is easier to design a 'one solution fits all' focal reducer - not changing the designed register distance by using the available space that normally is occupied by a flapping mirror - than it is to design a conventional teleconverter.

Note also that a 0.71x converter for m43 still cherrypicks the central portion of the image circle and crops out the often more troublesome original image circle periphery.

Of course, a plain m43 adapter crops away even more of the image circle periphery. But then keep in mind that a 16 MP m43 sensor has a pixel spacing comparable to a 64 MP full frame sensor. I don't think all Nikkors will have resolution like that.

David Thorpe claims a focal reducer even increases resolution:
https://youtu.be/qGIkq8b2iwg


Issues common to Viltrox 0.71x and K&F m43 adapter are that the lenses focused beyond infinity. The Viltrox has moveable optics, enabling adjustments to achieve infinity focus. The K&F has no such mechanism, but I guess it may be shimmed.

Harald

[Toby]

I did some investigation of the performance of Canon lenses on a Metabones speedbooster on a Sony A7RII. I was very surprised with the results, in that I could detect no degradation of the image in the center of the frame and very minimal softening at the extreme edges. Of course the Metabones costs about $500, and I'm sure the cheaper ones do not perform as well.

[Birna Rørslett]

I did some pretty extensive shooting with a Metabones Speed Booster and the 50mm f/1.2 Nikkor on a Panasonic GH-2 and found the results surprisingly good. One of the captures was used for a double-page spread with no complaints from the customer.

[Peter Forsell]

Note also that a 0.71x converter for m43 still cherrypicks the central portion of the image circle and crops out the often more troublesome original image circle periphery.

The speedbooster works just the opposite. It takes the larger image circle and squeezes it into a smaller image circle with higher light intensity. If the edges are troublesome, with a booster they are included, without they are cropped. What you described was how teleconverter works.

[Erik Lund]

Here is a link to a white paper by Brian Caldwell

https://www.metabones.com/assets/a/stories/Speed%20Booster%20White%20Paper.pdf

[Birna Rørslett]

The 0.71x speedbooster is the inverse of a 1.4X TC. The first adds a full stop, the latter makes the master lens 1 stop slower. The "speed" change is just a consequence of the corresponding decrease or increase in focal length, respectively, as the actual aperture size stays the same.

[HaraldH]

I guess I was not entirely clear. The net effect of a lens designed for full-frame and a 0.71x speed booster - on a m43 sensor - is stil a crop of the original full frame image. But less of a crop than without the speed booster.

A 0.5x speed booster will condense all of the full frame onto the smaller m43 sensor.